A discovered attack happens when you move one piece out of the way of another, revealing an attack that was hidden behind it the whole time. The piece that actually moves can do anything, even something that looks unrelated, since the real threat comes from the piece left behind.
This is especially dangerous when the moving piece also gives check itself, a discovered check. The opponent has to deal with the check from the piece that just moved, while the piece it uncovered is left free to win material completely undisturbed.
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White's bishop on g2 is aimed at the a8 corner, but the knight on e4 is in the way for now.
Which knight move wins the queen?
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